Moisture-delivering atomizers are widely used in the textile industry to produce a desired level of humidity in production spaces. When moisture delivered as a spray of very fine droplets evaporates into vapor, sensible heat is converted into latent heat, causing a temporary reduction in temperature in the sprayed area, and a compounded temporary increase in the relative humidity level is produced until the sprayed area has re-warmed. The atomizing devices are connected to a source of water typically at ambient temperature, and operate by use of compressed air admitted thereinto for blowing delivery of moisture in droplet form. The quantity of moisture being delivered generally cannot be modulated but rather is fixed by the design and adjustment of the atomizer. Generally the atomizers are turned on by opening a valve to admit compressed air to the atomizer, and turned off by closing the valve. Typically the air control valve is pneumatically actuated, by application of actuating air from a pneumatic relay, or from an electrically actuated pilot solenoid valve. Typically, as with any such on-off final control element, automatic control has been accomplished in the on-off control mode, a group of atomizers being turned on when a humidity sensor indicates a condition drier than is desired, and turned off when the sensor indicates a humidity level more moist than is desired.
The automatic control of atomizers operated in the on-off control mode has not been as responsive to conditions as desired in situations where fine control is necessary, the cyclic nature of the on-off mode producing swings in the humidity levels detrimental to production efficiency and product quality. In particular, in areas requiring higher humidity levels for best operating results, even a slight rise of humidity over the desired level can lead to condensate formation on walls, structures, or machinery, the temperature of which may be higher than the dew-point at the desired humidity level but lower than the dew-point if the humidity is allowed to exceed the desired level by only a few percent. The rate and amplitude of the cyclic changes noted when using the on-off control mode are influenced by a number of factors, including the sensitivity and response-time characteristics of the humidity sensor, the level of moisture demand, the total installed atomizer capacity, the varying heat loads encountered, the time lag for evaporation of the atomized moisture and re-warming of the air cooled thereby, the size of the space being served, and the rate of air circulation.
One attempt at reducing the extent of the problem is disclosed in my prior U.S. Pat. No. 3,460,557, which discloses the use of a time proportioning control mode rather than an on-off control mode, and is pneumatic or pneumatic-electric in its operation. The device of that patent operates by pneumatically creating a continuing succession of time cycles, and utilizes the humidity proportional pneumatic signal from a humidity sensor to divide each time cycle into on periods and off periods which inversely proportion the amount of moisture delivered to the signal level from the sensor. Many years of experience with many installations have demonstrated the distinct superiority of this type control over the on-off type control, but undesirable characteristics of this pneumatic time-proportioning device have been noted. Being pneumatic-electric, an installation of the device requires both the installation of pneumatic tubing as well as electric circuitry. Also required is an almost totally clean compressed air supply. The device also requires a pneumatically operated sensor, typically requiring a large cabinet equipped with forced circulation for the air whose humidity is being controlled. Pneumatic humidity sensors are invariably of the dimensionally changing sensing element type, which suffer from slow response, and considerable hysteresis. The pneumatic control is mechanical and is subject to failure from wear. It utilizes elastomeric diaphragms and dynamic seals in delicate and dependent relation to each other, and they are subject to failure from aging and from attack by contaminants in the air supply.
Also, an operational problem with the device of the aforementioned patent has been experienced on some occasions, the solutions to which in the pneumatic configuration have proved impractical. The problem originates with the sensing device response-time characteristic, which frequently does not instantly change its output signal with a change in the measured condition but lags behind the actual measured condition, and with the aforementioned compounded increase in relative humidity upon evaporation of atomized moisture droplets, and ensuing decrease upon re-warming. Particularly when the sensing device is operated at a high sensitivity, it has been noted that the output from the sensor will remain stationary for some time after a change in condition, then suddenly will change over a relatively large range, due to the stick-slip frictional characteristics of a mechanical device. This can cause rapid intermittent operation of the control signal to the air control valve, resulting in dribbling atomizers and undue wear of valves, atomizers, and controls. One method to avoid this problem is to reduce the sensitivity of the sensor, but this broadens the range of variation of the condition being controlled.
The present invention is directed toward eliminating the problems associated with previous devices. It is entirely solid state electronic, thus mechanical wear is eliminated. It does not require either compressed air or pneumatic tubing, hence, installation is simpler and less expensive. It may utilize any one of several types of electronic humidity sensors, which generally are far more responsive to changes in humidity and hence do not require forced ventilation and is well suited for use with a sensor of the type disclosed in my co-pending U.S. patent application Ser. No. 828,895 filed Aug. 29, 1977. This makes possible a relatively small sealed enclosure to protect the entire device from accumulations of dust, dirt and lint, with only the sensing element exposed. In addition the electronic configuration makes possible a straightforward solution to the delayed response problem mentioned above by locking in the humidity proportional signal at a selected point in each successive time cycle for use thereafter in the cycle.